1. Field of the Invention
The present application claims priority and contains subject matter related to Japanese Patent Application No. 2001-077282 filed in the Japanese Patent Office on Mar. 16, 2001, and the entire contents of which are hereby incorporated by reference.
The present invention relates to an image forming apparatus and an image forming method for selecting an optimal image space frequency for an output image, and a recording medium storing computer program instructions for performing selection of an optimal image space frequency for an output image.
2. Discussion of the Background
A dither method has been known as a technology for performing halftone processing for inputted image data. A dither method uses a threshold matrix composed of a plurality of thresholds (called a dither pattern) in halftone processing inputted image data, and the inputted image is binarized as black when the inputted image data is larger than a corresponding threshold of the matrix and white when the inputted image data is equal to or smaller than the corresponding threshold, thereby reproducing a halftone image.
In halftone processing of image data using a dither method, if the same threshold matrix is used for each image data of the same image quality (e.g., density) having different resolutions, it may occur that the image quality of an output image of each inputted image data is different from each other even though an original image of each inputted image data has the same image quality, causing a problem of inability of accurately reproducing an image of inputted image data.
A technology described in Japanese Patent No. 2832052 aims to solve the above-described problem, and is designed to acquire similar results even when inputted image data of the same image quality is different from each other in resolution by preparing a dither matrix for each resolution.
As enlargement/reduction methods used for enlarging and reducing characters, line images and the like, there have been known an SPC method, a logical sum method, a 9-division method, a high-speed projection method, a distance inverse proportion method, and the like. When these enlargement/reduction methods are applied to image data processed by a dither method, gradation information of an original image of the image data may be lost or moire fringes may be generated to deteriorate the quality of an output image of the image data.
In relation to the above-described problem in enlargement/reduction of dither-processed image data, Japanese Patent Application Laid-Open Publication No. 216476/1987 describes a technology, in which a group of original pixels located near a transformed pixel acquired by position change are selected as reference pixels, a density of the transformed pixel is obtained from binarized patterns of the selected reference pixels, and then data of the obtained density is binarized according to an organized dither method. According to the technology described in Japanese Patent Application Laid-Open Publication No. 216476/1987, after transformed pixels are acquired by position changing carried out according to a specified transformation magnification, a group of original pixels is selected as reference pixels for each of the transformed pixels, the number of black pixels among the reference pixels is specified as a density of the transformed pixel, and the acquired density of the transformed pixel is binarized by an organized dither method.
The use of the technology described in Japanese Patent Application No. 216476/1987 may solve the problem of image quality deterioration generated by the application of a SPC method or the like for enlargement/reduction of dither-processed image data. However, in enlargement, transformed pixels using the same group of original pixels as the reference pixels are processed assuming that they are similar in density. Consequently, edge portions of an output image of the dither-processed image data may be blurred.
In relation to the above-described problem of image quality deterioration following enlargement/reduction of dither-processed image data, another technology has been proposed in Japanese Patent No. 2899304 to selectively subject dither-processed image data generated according to an organized dither method to a second dithering process functioning as an aperture for the dither-processed image data.
Generally, various objects for processing are laid out in a document, such as images (e.g., bit-map images), characters containing character information and attribute information for the characters such as one regarding a character size or a font, and graphics containing coordinate information inputted by using a pointing device and attribute information for the graphics. Each of the objects for processing is edited and processed by manipulation of each attribute information, so that the document is formed. The formed document is printed and outputted by a printer.
In the above-described printing and outputting process of a document, a resizing process such as interpolation and thinning (pixel skipping) is carried out for images to be matched with a predetermined printing output size, thereby generating image data for printing. For characters and graphics, bit-map images thereof are generated with an outputting resolution based on data of the characters or graphics after having been laid out. The bit-map images of resized images and those of characters and graphics are synthesized to form a printing image.
Thus, because data of a document includes information of a layout of the document and a printing image of each object for processing, laying out or printing of the printing image can be carried out by manipulating the information included in the data of the document, and it is not necessary, for example, to read image data of an image, which has been read before by an image scanner or the like, again in laying out or printing and outputting of the image data.
Among various objects for processing included in a document, the object for processing regarding characters or graphics does not require a large storage capacity because the object only includes information for defining shapes of characters and graphics and attribute information for the characters and graphics such as one regarding color or filling pattern thereof. On the other hand, the object for processing regarding a bit-map image requires an information storage capacity considerably larger than that for the object regarding characters or graphics because information of a bit-map image is based on a pixel unit.
In addition, as for the object for processing regarding characters or graphics, even when sizes of the characters or graphics are changed for example in printing and outputting of the characters or graphics, it is only necessary to change information defining shapes of the characters or graphics according to a printing output size. Accordingly, the quantity of data of an object regarding characters or graphics does not change significantly even when sizes of the characters or graphics are changed.
On the other hand, as for the object for processing regarding a bit-map image, when the size of an output image is increased, data of the bit-map image is adjusted by interpolation. Consequently, resolution of the output image is lowered, for example, causing a reduction in the image quality. To prevent such a reduction in image quality, an image must be read with a resolution suitable for printing and outputting of an output image, which leads to increasing the size of resulting image data. If an image is thus read beforehand using a relatively large data size according to the size of an output image, image data of the large data size must be processed in laying out work on a computer screen not related to the size of an output image, causing considerable slowing down in screen updating or laying out work on the computer. Thus, for carrying out quick laying out work on the computer, the computer requires a memory of a relatively large capacity and a CPU of a relatively high-speed operation.
In relation to the above-described problem, a technology is described in Japanese Patent Application Laid-Open Publication No. 2000-76472. According to this technology, image data of an image read with a low resolution is used in laying out work of the image, and in printing and outputting of the image, based on information of an original image read with the low resolution and information of the size of an output image, a resolution optimal for printing and outputting is set at an image reading device, the image is read again by the image reading device with the set resolution, and image data thus obtained is used.
Generally, when an image is seen, a certain angle of visual field is set. Further, a visual distance is set relatively small when a relatively small image is seen (e.g., an image at the distance “a” in FIG. 3), while the visual distance is set relatively large when a relatively large image is seen (e.g., an image at the distance “b” in FIG. 3).
Also, depending on the type of a design or the kind of an image or graphic to be seen, the required image space frequency varies. For example, the required image space frequency varies between an image of a human face and an image of a simple graphic or a design painted by one color.
Further, it is known that when performance of an image forming apparatus affects quality of a formed image and when image data is halftone processed with a dither process in the image forming apparatus, the quality of an output image may be more stable when the number of dither lines of a dither pattern (matrix) of the dither process is relatively small than when the number of dither lines of the dither pattern is relatively large. Generally, a processing method is employed, where the number of dither lines of a dither pattern is set relatively small to improve granularity or graininess and uniformity of an output image, and the number of dither lines is set relatively large to improve definition of the output image. However, in the above-described processing method, because no consideration has been given to a relation between a size of an output image and a visual distance for the output image, it often occurs that the quality of the output image is not satisfactory.